February 22, 2011

Physicists Discover Quantum Law of Protein Folding

The famous Arrhenius relationship states that things happen faster as they got hotter. In chemistry, that’s generally true but there’s an important exception: the speed at which proteins fold into their functional shape.

It’s easy to think that proteins ought to fold more quickly as they cool down and then unfold more quickly as they heat up. But the actual relationship is both nonlinear and asymmetric, meaning that unfolding is not the reverse of folding.

Molecular biologists have put forward various mechanisms to explain this, such as the nonlinear interaction between water and hydrophobic parts of proteins. But none of these are very convincing.

That looks set to change with the work of Liaofu Luo at the Inner Mongolia University and Jun Lu at the Inner Mongolia University of Technology, both in China. They say that the way folding depends on temperature all becomes clear as soon as you take quantum mechanics into account.

First, a little background on protein folding. Proteins are long chains of amino acids that become biologically active only when they fold into specific, highly complex shapes. The puzzle is how proteins do this so quickly when they have so many possible configurations to choose from.

To put this in perspective, a relatively small protein of only 100 amino acids can take some 10^100 different configurations. If it tried these shapes at the rate of 100 billion a second, it would take longer than the age of the universe to find the correct one. Just how these molecules do the job in nanoseconds, nobody knows.

What they do know, however, is that the rate at which they fold is highly sensitive to temperature and biologists have a significant amount of data showing exactly how these rates vary. Plotting these data leads to various unexpected curves.

Today, Luo and Lo say these curves can be easily explained if the process of folding is a quantum affair. By conventional thinking, a chain of amino acids can only change from one shape to another by mechanically passing though various shapes in between.

But Luo and Lo say that if this process were a quantum one, the shape could change by quantum transition, meaning that the protein could ‘jump’ from one shape to another without necessarily forming the shapes in between.

Luo and Lo explore this idea using a mathematical model of how this would work and then derive equations that describe how the rate of “quantum folding” would change with temperature. Finally they fit the rpedictions of their model to some real world experiments.

Their astonishing result is that this quantum transition model fits the folding curves of 15 different proteins and even explains the difference in folding and unfolding rates of the same proteins.

That’s a significant breakthrough. Luo and Lo’s equations amount to the first universal laws of protein folding. That’s the equivalent in biology to something like the thermodynamic laws in physics.

If quantum mechanics plays a key role in protein folding, then there can be little question of its importance in the workings of other cellular machines. It can only be a matter of time before the floodgates open for quantum biologists.